Pollution May Slow Warming; Cleaner Air May Speed It, Study Says

Pollution may be slowing global warming, researchers are reporting today, and a cleaner environment may soon speed it up.

Writing in the journal Nature, an international scientific team provides evidence suggesting that a reduction in haze from human causes may accelerate warming of the earth's atmosphere. The researchers said pollutants had held down the rate of global warming by absorbing and scattering sunlight.

"If people clean up the air, more warming will come blazing through," Jim Coakley, a professor of atmospheric sciences at Oregon State University in Corvallis, said yesterday in a telephone interview. Nature selected Dr. Coakley to write a commentary on the study.

The scientists, who work for government agencies in Britain and the United States, made the finding after adding satellite-based measurements of haze to computer models estimating the consequences of industrial emissions of aerosols, or airborne particles.

Haze scatters and absorbs some sunlight, keeping it from reaching the ground, and this cooling effect is stronger than many scientists had believed, the study says. The cooling offsets about one-third of the warming from the use of fossil fuels and other manmade causes, the study says.

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Global temperatures are already about seven-tenths of a degree Fahrenheit higher than they were in the 1880's, Dr. Coakley said, and expected further warming may mean that within decades, summers will be about a month longer than they are now.

The new estimate of the cooling effect of haze is at the high end of ranges cited by the Intergovernmental Panel on Climate Change, a study group created by the United Nations Environment Program and the World Meteorological Organization.

But Dr. Coakley noted that the science was still complicated and that other factors remained to be measured, including the cooling effect of sunlight reflections from water droplets associated with haze, as distinct from the haze itself. Reducing haze would eliminate the droplets and remove yet another source of cooling, he said.

"This is a brave effort," he said of the new report. "But let's see what others come up with now."

In 1995, Paul Crutzen, Mario Molina and F. Sherwood Rowland received the Nobel Prize in Chemistry for explaining how the ozone shield, which screens out harmful radiation, is damaged by industrial gases.